1. Field of the Invention
The present invention relates to a control system for a commutatorless motor, in which a synchronous motor is driven by a frequency converter comprising thyristors, or more in particular to a control sytem for a commutatorless motor, which is capable of stable change-over between motoring operation and regenerative operation and between forward operation and reverse operation.
2. Description of the Prior Art
A commutatorless motor, as well known, is a variable speed motor which is a combination of a synchronous motor and a frequency converter comprising thyristors for controlling the motor and which has neither commutator nor brush. The commutatorless motor is advantageous in that it has a very wide range of controllable speeds and that its lack of commutator and brush facilitate maintenance and inspection.
In the commutatorless motor, the four-quadrant operation including forward motoring operation and regenerative operation and reversing motoring operation and regenerative operation is available by changing the firing angle on the motor side of the frequency converter. In such a commutatorless motor, the firing angle on the motor side (angle of advance) is required to be changed in accordance with the operating conditions. This is for the reason that at the time of starting the motor, the angle of advance is lessened in order to attain the maximum torque, while at the time of loaded operation, the angle of advance is required to be increased in accordance with the load current in order to prevent any commutation failure which in turn is attributable to lack of the margin angle of commutation.
The prior art for overcoming such a problem is disclosed, for example, in the U.S. Pat. No. 3,778,691 entitled "Control Circuit for an Electric Machine Having an Electronic Commutator." According to this U.S. Pat. No. 3,778,691, a fixed pulse is obtained through combination of three position signals with the width of 180 electrical degrees and phase difference of 120 degrees and inverse position signals resulting from inversion of the position signals. At the same time, a speed signal is integrated for a predetermined period associated with the combination of the postion signals, so that a phase-shifted pulse is produced by comparing the integrated speed signal with a phase control signal. The thyristors are fired by the fixed pulse or the phase-shifted pulse, whichever is produced earlier. This U.S. Pat. No. 3,778,691 enables the maximum torque to be achieved at the time of starting on the one hand and the commutation failure to be prevented on the other hand.
The phase rotation at the time of forward operation is reverse to that at the time of reversing operation, thus making it necessary to change the combination of the position signals. This switching operation is usually performed by selecting AND gates in response to a forward operation command signal and a reversing operation command signal. This switching operation is made under the condition that the motor speed is zero. However, whether the motor speed is zero or not cannot be strictly decided but with a certain margin. The result is that at the time of switching, an irregular firing pulse is generated, thus leading to a fault such as the short-circuiting of the power supply. Further, at the time of switching from forward to reversing operation, the forward rotation is undesirably accelerated again if the switching operation is carried out in the speed range of forward operation. As a result, stable change-over between forward and reversing operations is impossible according to U.S. Pat. No. 3,778.691.